Color-shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit the property of changing hue upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The primary method used to achieve such color-shifting colorants is to disperse small flakes, which are typically composed of multiple layers of thin films having particular optical characteristics, throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object. Color switching pigments appear to change color for example from a dark green to a light green, or from a light blue to a dark blue. Color switching pigments are described in U.S. Pat. No. 6,150,022 in the name of Coulter et al. Color switching pigments consist of bright metal flakes that are substantially reflective disposed in a liquid carrier vehicle that includes a dye. For example when a blue carrier vehicle is used, the flakes have a range of color from light to dark blue when they switch color upon a change in viewing angle.
Diffraction patterns and embossments, and the related field of holographs, have begun to find wide-ranging practical applications due to their aesthetic and utilitarian visual effects. For all intents and purposes, a diffraction pattern, whether embossed, etched or inked, is be understood to be a marked region. A marked region is to be understood to be a region having some form of indicia thereon, whether inked or stamped or etched. One very desirable decorative effect is the iridescent visual effect created by a diffraction grating. This striking visual effect occurs when ambient light is diffracted into its color components by reflection from the diffraction grating. In general, diffraction gratings are essentially repetitive structures made of lines or grooves in a material to form a peak and trough structure. Desired optical effects within the visible spectrum occur when diffraction gratings have regularly spaced grooves in the range of hundreds to thousands of lines per millimeter on a reflective surface.
Diffraction grating technology has been employed in the formation of two-dimensional holographic patterns which create the illusion of a three-dimensional image to an observer. Three-dimensional holograms have also been developed based on differences in refractive indices in a polymer using crossed laser beams, including one reference beam and one object beam. Such holograms are called volume holograms or 3D holograms. Furthermore, the use of holographic images on various objects to discourage counterfeiting has found widespread application.
There currently exist several applications for surfaces embossed with holographic patterns which range from decorative packaging such as gift wrap, to security documents such as bank notes and credit cards. Two-dimensional holograms typically utilize diffraction patterns which have been formed on a plastic surface. In some cases, a holographic image which has been embossed on such a surface can be visible without further processing; however, it is generally necessary to coat a reflective layer upon the embossed surface, typically a thin metal layer such as aluminum in order to achieve maximum optical effects. The reflective layer substantially increases the visibility of the diffraction pattern embossment.
Every type of first order diffraction structure, including conventional holograms and grating images, has a major shortcoming even if encapsulated in a rigid plastic. When diffuse light sources, such as ordinary room lights or an overcast sky, are used to illuminate the holographic image, all diffraction orders expand and overlap so that the diffraction colors are lost and not much of the visual information contained in the hologram is revealed. What is typically seen is only a silver colored reflection from the embossed surface and all such devices look silvery or pastel, at best, under such viewing conditions. Thus, holographic images generally require direct specular illumination in order to be visualized. This means that for best viewing results, the illuminating light must be incident at the same angle as the viewing angle.
Since the use of security holograms has found widespread application, there exists a substantial incentive for counterfeiters to reproduce holograms which are frequently used in credit cards, banknotes, and the like. Thus, a hurdle that security holograms must overcome to be truly secure, is the ease at which such holograms can be counterfeited. One step and two step optical copying, direct mechanical copying and even re-origination have been extensively discussed over the Internet. Various ways to counteract these methods have been explored but none of the countermeasures, taken alone, has been found to be an effective deterrent.
One method used to reproduce holograms is to scan a laser beam across the embossed surface and optically record the reflected beam on a layer of a material such as a photopolymerizable polymer. The original pattern can subsequently be reproduced as a counterfeit. Another method is to remove the protective covering material from the embossed metal surface by ion etching, and then when the embossed metal surface is exposed, a layer of metal such as silver (or any other easily releasable layer) can be deposited. This is followed by deposition of a layer of nickel, which is subsequently released to form a counterfeiting embossing shim.
Due to the level of sophistication of counterfeiting methods, it has become necessary to develop more advanced security measures. One approach, disclosed in U.S. Pat. Nos. 5,624,076 and 5,672,410 to Miekka et al., where embossed metal particles or optical stack flakes are used to produce a holographic image pattern.
A further problem with security holograms is that it is difficult for most people to identify and recollect the respective images produced by such holograms for verification purposes. The ability of the average person to authenticate a security hologram conclusively is compromised by the complexity of its features and by confusion with decorative diffractive packaging. Thus, most people tend to confirm the presence of such a security device rather than verifying the actual image. This provides the opportunity for the use of poor counterfeits or the substitution of commercial holograms for the genuine security hologram.
In other efforts to thwart counterfeiters, the hologram industry has resorted to using more complex images such as producing multiple images as the security device is rotated. These enhanced images provide the observer with a high level of “flash” or aesthetic appeal. Unfortunately, this added complexity does not confer added security because this complex imagery is hard to communicate and recollection of such imagery is difficult, if not impossible, to remember.
It would therefore be of substantial advantage to develop improved security products which provide enhanced viewing qualities in various lighting conditions, especially in diffuse lighting, and which are usable in various security applications to make counterfeiting more difficult.
Security articles having diffractive surfaces and color-shifting backgrounds are described U.S. patent application Ser. Nos. 20040105963 A1, 20040101676 A1, 20040094850 A1, and 20040081807 A1. Such security devices include a transparent holographic substrate coated with a color-shifting layer on the side opposite to the holographic embossing. The color-shifting optical coating provides an observable color shift as the angle of incident light, or viewing angle, changes. The color-shifting coating can be fabricated by vacuum deposition of an optical interference structure onto the corresponding surface of the substrate, by spraying of a paint containing color-shifting pigment, or by printing ink as by flexographic, gravure or Intaglio means.
A patterned layer of a reflective material might be applied over predetermined portions of the holographic substrate to form alphanumeric characters, bar codes, pictorial or graphic designs as described in WO 2005/026848 A2. To produce such, a highly reflective material needs to be deposited on the top of the holographic substrate and etched out from predetermined portions of the substrate. As a result of demetalizing these areas of the substrate, where the metal was etched out, they become essentially transparent and the holographic effect there becomes almost invisible. In contrast, the portions of the substrate where the reflective metal was left on the surface in different predetermined shapes, maintain visible holographic properties.
Color-shifting coatings can be applied to such a demetalized structure in different ways. It can be applied to the side of the substrate opposite to the embossed side. In this manner the coating becomes visible through transparent demetalized portions of the substrate. Alternatively the color-shifting coating can be applied on the top of embossed side. The coating and patterned holographic elements become visible through the transparent substrate when the substrate is flipped over. This combination of hologram substrate and a color-shifting coating is called a “chromagram”. General concept of chromagrams can be readily understood with reference to FIGS. 1 through 5.
Demetalized holograms are more difficult to counterfeit since one not only has to make the hologram but also demetalize an intricate pattern in register with the holographic pattern.
It is an object of this invention, to provide an image that can be used as a security device, that is very difficult for counterfeiters to copy, and that can readily be authenticated.
It is a further object of the invention to provide a security device that offers a high degree of security while at same time providing considerable visual appeal.